Apply "silent code cleanup" from IntelliJ

[Baltoli]

This PR applies all automatic cleanups that IntelliJ is confident enough to apply silently with no further user input; a rough summary of the different kinds of change are:

• Removal of redundant casts (T t = (T) v where v already has type T).
• Removal of unnecessary calls to toString()
• Insertion of final where possible
• Collapsing of redundant boolean conditions if (cond) return true else return false
• Syntactic fixes (;; etc.)
• Use of StandardCharsets.UTF_8 to simplify exception checks when encoding strings
• Removal of redundant explicit key checks when removing values from hashmaps

As ever, I'm happy to bikeshed these changes - I can redo the analysis and disable some subset of the checks if anyone has objections to the specific results we get.

To avoid merge conflicts, this PR is based on #3746 and should only be merged when that PR is merged.

[radumereuta]

Overall, it looks good. I added a question though. Please double check.